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WASHINGTON — Exposure to ultrasound can affect fetal brain development, a new study suggests. But researchers say the findings, in mice, should not discourage pregnant women from having ultrasound scans for medical reasons.

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When pregnant mice were exposed to ultrasound, a small number of nerve cells in the developing brains of their fetuses failed to extend correctly in the cerebral cortex.

"Our study in mice does not mean that use of ultrasound on human fetuses for appropriate diagnostic and medical purposes should be abandoned," said lead researcher Pasko Rakic, chairman of the neurobiology department at Yale University School of Medicine.

However, he added in a telephone interview, women should avoid unnecessary ultrasound scans until more research has been done.

Dr. Joshua Copel, president-elect of the American Institute of Ultrasound Medicine, said his organization tries to discourage "entertainment" ultrasound, but considers sonograms important when there is a medical benefit.

"Anytime we're doing an ultrasound we have to think of risk versus benefit. What clinical question are we trying to answer," Copel said in a telephone interview. "It may be very important to know the exact dating of pregnancy, it's certainly helpful to know the anatomy of the fetus, but we shouldn't be holding a transducer on mom's abdomen for hours and hours and hours."

Rakic's paper said that while the effects of ultrasound in human brain development are not yet known, there are disorders thought to be the result of misplacement of brain cells during their development.

Their report is in Tuesday's edition of Proceedings of the National Academy of Sciences.

Early ultrasound scans are done to determine the exact week of the pregnancy and they are also done later to check for anatomical defects and other problems.

Ultrasounds for entertainment
However, some expectant parents have sought scans to save as keepsakes even when they were not medically necessary, a practice the Food and Drug Administration discourages.

The Institute of Ultrasound Medicine was particularly concerned last year when it was announced that actor Tom Cruise had purchased an ultrasound machine for his pregnant fiancee, Katie Holmes, so they could do their own sonograms.

"Purchase of an ultrasound machine for private, at home use entails inappropriate operation of a prescription medical device designed for diagnostic use by a trained medical professional," the group said in a statement issued at the time.

Of mice and women
Copel, a professor of obstetrics and gynecology at Yale University School of Medicine, did point out that there are large differences between scanning mice and scanning people.

For example, because of their size, the distance between the scanner and the fetus is larger in people than mice, which reduces the intensity of the ultrasound. In addition, he said, the density of the cranial bones in a human baby is more than that of a tiny mouse, which further reduces exposure to the scan.

The paper noted that the developmental period of these brain cells is much longer in humans than in mice, so that exposure would be a smaller percentage of their developmental period.

However, it also pointed out that brain cell development in people is more complex and there are more cells developing, which could increase the chances of some going astray.

In Rakic's study, pregnant mice were exposed to ultrasound for various amounts of time ranging from a total exposure of 5 minutes to 420 minutes. After the baby mice were born their brains were studied and compared with those of mice whose mothers had not been exposed to ultrasound.

The study of 335 mice concluded that in those whose mothers were exposed to a total of 30 minutes or more, "a small but statistically significant number" of brain cells failed to grow into their proper position and remained scattered in incorrect parts of the brain. The number of affected cells increased with longer exposures.

The research was funded by the National Institute of Neurological Disorders and Stroke.

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